AdvFilterCommandParser.java: Check for valid operators when parsing

[RyanWarwick]

Now checks if user input contains valid operator and shows Invalid Command text etc otherwise. Resolves #131.

Fixed comparison using operators and between Double and String types. In particular: resolves #135 and resolves #136

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 51.47059% with 33 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...seedu/address/logic/commands/AdvFilterCommand.java	54.09%	20 Missing and 8 partials :warning:
...u/address/logic/parser/AdvFilterCommandParser.java	28.57%	2 Missing and 3 partials :warning:

Files with missing lines	Coverage Δ	Complexity Δ
...u/address/logic/parser/AdvFilterCommandParser.java	63.15% <28.57%> (-10.18%)	4.00 <0.00> (ø)
...seedu/address/logic/commands/AdvFilterCommand.java	61.76% <54.09%> (-6.30%)	27.00 <6.00> (-2.00)